Many people refer to it incorrectly as its "dark" side, but the far side of our moon…
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The GRAIL mission relies on two washing-machine-sized satellites (named Ebb and Flow) to map not only the Moon's surface, but its interior. The orbiting pair loops around the Moon in tandem, with one satellite constantly tailing the other like a sighthound coursing game. Variations in the Moon's gravitational field cause the satellites to grow closer or further apart. Equipment onboard the satellites translates these variations in speed and distance into not just the highest resolution map of the Moon ever made, but the highest resolution gravity field map of any celestial body.

The gravity field map reveals an abundance of features never before seen in detail, such as tectonic structures, volcanic landforms, basin rings, crater central peaks and numerous simple, bowl-shaped craters. Data also show the moon's gravity field is unlike that of any terrestrial planet in our solar system.

According to GRAIL Principle Investigator Maria Zuber, the Moon, more than any other celestial body, "wears its gravity field on its sleeve... When we see a notable change in the gravity field, we can sync up this change with surface topography features such as craters, rilles or mountains." Everything from the Moon's impact history to the formational secrets of its deep geology are writ large in the highly variable tug of its gravitational field.

Wired's Nadia Drake provides a tidy summary of some of the GRAIL team's preliminary findings:

GRAIL's measurements reveal that gravity fields are strongly associated with features on the lunar surface, such as impact basin rings, central crater peaks, and volcanic landforms. The moon's crust is also thinner than expected, averaging between 18 and 25 miles deep. Such a thin crust suggests that the moon's chemical composition is similar to Earth's.

And, for the first billion years of its life, the early moon was expanding. GRAIL data returned evidence of long, magmatic intrusions into the lower crust – sites where magma seeped into cracks and solidified. These intrusions, totaling 3,300 miles in length, suggest the young moon warmed and expanded for about a billion years, with its radius growing between 0.3 and 3 miles.

This clip shows the variations in the lunar gravity field. Warmer colors refer to mass excesses and areas of higher local gravity, cooler colors to mass deficiencies and areas of lower local gravity. Note the heightened level of small-scale detail on the far side, owing to the fact that the Moon's far side has more small craters.

Data such as these provide crucial insights not just into the Moon's orgins, but Earth's. These findings, notes planetary scientist Mark Wieczorek, a member of the GRAIL team, are "consistent with a hypothesis that the moon is most likely derived from materials that come from the Earth, following a giant impact event." (Read more about recent research on the Moon's explosive beginnings here.)

Remember, this is just the first wave of GRAIL results, all from the primary science mission — Zuber's team has a lot of work ahead of it, and — we're willing to bet — many more impressive discoveries.

"[The GRAIL mission] will improve our knowledge of the Moon's nearside gravity by more than 100 times over what was previously known, and by more than 1,000 times over what was known on the far-side," said Zuber shortly after Ebb and Flow safely entered the Moon's orbit almost one year ago.

"In science, when we can improve measurements by a factor of two, we usually learn an awful lot," said Zuber.

"But when we improve measurements by orders of magnitude, the kind of science that we do is actually transformative."